This invention relates to liquid treating apparatus and more particularly to apparatus for treating waste water, including both domestic sewage and those industrial wastes susceptible to aerobic treatment. The system is particularly suited to providing treatment to waste water which has been directly discharged from a home. These discharges have characteristics which make them more difficult to handle than are the amalgamated discharges from several homes. However, the total system is also equally applicable to handling loads of any capacity which are susceptible to the biological mode of treatment. Also, certain components of my improved system are adaptable for use to increase the operational efficiency of other types of aerobic, or of anaerobic, waste water treatment systems.
My invention is an improvement on conventional methods known as `activated sludge` which consists of the aeration of waste water; the separation of solids and liquids at the end of the aeration period; the discharge of the liquid; and, the return of some or all of the separated solids to the aeration zone.
Some activated sludge secondary treatment systems have the aeration chamber preceded by primary settling chambers to remove much of the settleable and floatable materials. These in turn may be preceded by other devices, such as grit chambers, comminutors and the like to remove large materials or to shread materials into fine particle sizes whereby they are more accessible to bacteria and so that they will not clog the various treatment elements of the system. Accordingly, the function of the comminutor is very important to the effectiveness of prior art treatment systems. Mechanical comminutors are expensive, require physical cleaning and disposal of refuse and are subject to frequent break-down. Also, they do not affect hair and will pass many objects, such as adhesive tapes, small plastic bags, rubber articles, and the like, which in turn frequently pass into and through the aeration phase of the system. Such materials, particularly hair in large quantities, break down very slowly in all phases of the treatment system and cause problems with the various elements of the settling chambers and digesters.
In total secondary systems which employ separate primary clarifiers and digesters, the waste water is exposed to aeration for too short a time for oxidation of materials such as heavy paper, cloth, hair, grease balls, rubber and seeds. Also, these materials are not broken down in systems of the extended aeration type which usually provide 24 hours aeration. Accordingly, in both systems, these materials can pass into the settling chambers where they can interfere with proper operation of the various return sludge or skimmer devices or pass out in the effluent. Prior attempts to solve this problem has required large diameter sludge and skimmer lines.
The principal problem encountered in prior art systems, insofar as obtaining an effluent low in suspended solids and biochemical oxygen demand, is to obtain good settling in the final chamber. Major factors involved in accomplishing this are: to return the maximum amount of the micro-organisms for reseeding purposes; to achieve the maximum in bacteria age; to remove all floatables before they can be discharged; to maintain sufficient dissolved oxygen throughout the settling chamber to support not only bacteria but the higher forms of life, such as protozoa and rotifers, which prey on bacteria; and to achieve maximum quiescence so that settleable material will not be held in suspension. This need for quiescence prevents the introduction of air or oxygen in the settling chamber and requires an extensive settling or holding time. Extensive holding time increases the oxygen demand, promotes septicity and results in floating sludge. A long holding period at low dissolved oxygen conditions results in an extended period at low food levels whereby there is a dropoff of the primary bacteria which stabilize the organic material in the head end of the aeration chamber. This in turn results in a long recovery period to handle the waste encountered when the sludge is returned to the aeration tank whereby there is a loss in process efficiency. in
It is necessary to maintain a high ratio of mixed liquor and suspended solids in the aeration chamber to accelerate the removal of organic materials and to reduce the susceptibility of the organisms to shock loads. This is achieved by maximizing the age and numbers of the organisms which in turn is directly related to the degree of sludge return achieved. Also, the sludge which is not returned must either be removed mechanically or allowed to go out in the effluent. Prior art means for returning sludge and skimmings, including the large diameter lines required, causes currents in the settling chamber and stirs up settled materials whereby such means are only operated intermittently. Also, the flow of liquid into the settling chamber, resulting from input into the system and from the liquid returned by the sludge and skimmer systems, in turn produces currents which keep materials on suspension. On the other hand, it is necessary to provide for frequent exchange of liquid between the aeration chamber and the settling chamber in order to provide adequate dissolved oxygen in the settling chamber to support aerobic conditions.
The need to have sufficient settling time and the need for quiescence in prior art equipment must be sacrificed somewhat to obtain at least some sludge return for reseeding purposes. Current equipment usually calls for at least 25% sludge return. Large systems are usually restricted to less than 50% sludge return. While small systems may be able to reach a limit of 200% sludge return from a cost basis, they are prevented from doing so because of the adverse effect on settling. Accordingly, process efficiency in aeration in prior art equipment is decreased, including failure to achieve nitrification, thus failure to achieve denitrification; creation of excess sludge or sludge blankets in the clarifier; poor ratio of biochemical oxygen demand and suspended solids removal; susceptibility to shock loading; a loss of uniformity in the bacteria species; and insufficient dissolved oxygen in the clarifier to support aerobic organism activities. Some components of waste water require an extensive exposure before they can become stabilized; a fraction of the synthesized cell initially is biologically inert and is not metabolized in prior art systems. Their biochemical demand is thus passed to the receiving waters or soil mantel. Furthermore, some components of waste water, such as hair, grease, seeds and the like, require an extensive exposure to aerobic organisms before they can be stabilized. Accordingly, the retention of these slow-to-stabilize components and materials as well as paper, cloth, wood and the like until completely degraded and metabolized makes them available as sources of food during low feed periods. One of the problems of small systems or systems fed intermittently is the drop in available food during periods of no flow or low flow, with corresponding changes in organisms and efficiency. The major organic loading food supply in an extended aeration system is the sludge itself. Accordingly, the heavier the mixed liquor-suspended solids in the system, the more consistent is the food supply for the organisms and the more there is available during low flow periods.